The present invention relates generally to the detection of the time of arrival of signal pulses and measuring their amplitudes. This problem arises in many fields and contexts. For timing, a small set of examples include: medicine, where coincidence between photons is the core component of PET scanning; nuclear physics, where time of arrival is used to determine neutron energies; and LIDAR, where range to target is found from total photon travel time. This list is far from complete and is presented merely to suggest the breadth of applications. We note that, commonly, timing measurements fall into two classes.
In the first class, the time of arrival is compared to some “start” signal and the measured interval carries the information of interest (e.g., LIDAR, where “start” is the emission of the laser pulse and the time interval represents the distance to the target). An important subclass is time-of-flight (TOF), where the interval measures the time for a particle to traverse a known distance. This is the way neutron energies are measured, with the start coming from a nuclear decay and the stop from a proton ejected in a scintillator by the neutron. Dividing the trajectory distance by the TOF gives the neutron's velocity and hence its energy. The amplitude of the proton-generated pulse carries no information about the neutron's energy and is not of interest.
In the second class, time differences between multiple events are measured and compared and only those pairs that occur within a preset time difference (the “coincidence window”) represent valid events for analysis (e.g., PET, where pairs of gamma-rays emitted from the same positron decay are found thusly). For amplitude measurement, a common example is the determination of energies of x-rays and gamma-rays absorbed in a variety of detectors, where accuracies of 0.1 to 1-2 percent may be required, depending upon the application. Here the time of arrival is not of interest. We therefore see that, depending upon the application, the time-of-arrival, the amplitude, or both may be of interest. Further, while this invention was developed in the context of the detection of photons in photomultiplier tubes, its range of application will be much broader.